When a video signal picked up by, for example, a video camera is recorded by a video tape recorder (VTR), in order to operate these two apparatus in synchronism with each other, if the VTR, for example, is used as a master apparatus and the video camera as a slave apparatus, data signals containing a control signal, a mode signal, an answer signal and the like must be interchanged therebetween. In such case, in the prior art, as shown in FIG. 1, two signal lines 3 and 4 are generally interconnected between a master apparatus 1 and a slave apparatus 2. Then, in order that through the first signal line 3, a clock signal S1 (FIG. 2A) is transmitted from the master apparatus 1 to the slave apparatus 2 and that in synchronism with this clock signal S1, a data signal S2 (FIG. 2B) is transmitted from the master apparatus 1 to the slave apparatus 2, a data signal DT12, of time points from t.sub.1 to t.sub.2 is transmitted from the master 1 to the slave 2 or, in the reverse direction, a data signal DT21 of time points from t.sub.3 to t.sub. 4 is transmitted from the slave apparatus 2 to the master apparatus 1. In the case of FIG. 1, the level of the signal line 3 for the clock signal S1 is always maintained at logic level "1" so that during the interval t.sub.1 to t.sub.2 or the interval t.sub.3 to t.sub.4 in which each bit of the data signal DT12 or DT21 of, for example, 4 bits is transmitted, the level of this signal line falls down to logic "0" with a duty ratio of 1/2.
As described above, in the prior art, in order that the data signal S2 may be positively interchanged between the two apparatus without being dropped, it is important that while the two apparatus are being operated in synchronism with each other by the common clock signal S1, the data is interchanged bit by bit therebetween so as to prevent the transmission and reception of the data signal S2 from being overlapped in time.
However, according to the circuit arrangement of FIG. 1, the two signal lines 3 and 4 must be provided and hence this puts a restriction on simplifying the circuit arrangement of the control system as the total system including two apparatus.